feat: Redis幂等锁 + 关键端点防重复提交

## decorators.py
- 新增 @prevent_double_submit(lock_timeout=5) 装饰器
  Redis key: idem:{user_id}:{path}:{MD5(body)}
  锁存在→409, 不存在→setex→执行业务→delete
  Redis不可用时fail-open降级放行

## purchase.py
- POST /purchase (创建): +@prevent_double_submit
- PATCH /purchase/<id>/approve (审批): +@prevent_double_submit

## outbound.py
- POST /outbound (出库): +@prevent_double_submit

## transactions.py
- POST /borrow/dispatch (借库扣减): +@prevent_double_submit

inventory-backend/app/api/v1/outbound.py

@@ -1,7 +1,7 @@
 from flask import Blueprint, request, jsonify
 from app.services.outbound_service import OutboundService
 from flask_jwt_extended import jwt_required, get_jwt_identity, get_jwt
-from app.utils.decorators import permission_required, audit_log
+from app.utils.decorators import permission_required, audit_log, prevent_double_submit
 from app.services.auth_service import AuthService
 import traceback
 
@@ -110,6 +110,7 @@ def scan_barcode():
 # --------------------------------------------------------
 @outbound_bp.route('', methods=['POST'])
 @jwt_required()
+@prevent_double_submit(lock_timeout=5)
 @audit_log(
     module='出库管理',
     action='新增',